The goal of the proposed program will be to establish two new model cultured cell systems (derived from horse and pigeon tissues) in which the age of the donor animal can be correlated with the proliferative capacities of the cells derived from that donor. Emphasis will be placed on the primary isolation and in vitro cultivation of homogeneous mass cultures and clonal lines of vascular endothelial cells. Other cell types such as fibroblasts will be isolated from the same donors as controls. The proliferative characteristics and specialized functions of the endothelial cells (and to a certain extent the control cells) will be characterized as a function of donor age and population doubling level in vitro.